Hybrid electric vehicles (HEV's) utilize a combination of an internal combustion engine with an electric motor to provide the power needed to propel the vehicle. This arrangement provides improved fuel economy over a vehicle that has only an internal combustion engine. Clutches may be used to control the power flow and during transient vehicle operations, such as pulling up the engine or the electric motor, to provide smooth operation for the driver.
For example, the engine may be shutdown in an HEV during times that the engine operates inefficiently, or is not otherwise needed to propel the vehicle. In these situations, the electric motor is used to provide all of the power needed to propel the vehicle. When the driver power demand increases such that the electric motor can no longer provide enough power to meet the demand, or if the battery state of charge (SOC) drops below a certain level, the engine needs to start quickly and smoothly in a manner that is nearly transparent to the driver to provide additional power for the vehicle. When the vehicle is being propelled by the motor, and an engine start is requested, it is desirable to control the powertrain, i.e. the engine, motor, transmission, etc., such that the flow of torque to the vehicle wheels is not substantially disrupted. A clutch may be slipped during engine start to isolate torque disturbances from the driveline. However, the present disclosure recognizes that inertia energy added to the rotating components of the system while the clutch is slipping must be dissipated quickly after the starting event without introducing discernible torque disturbances. Therefore, a need exists for a vehicle, and a method for controlling clutch engaging events in a vehicle, that reduces or eliminates driveline torque disturbances.